Modern solid-state electronic devices, such as transistors and other circuits and switches, rely on high-quality, easily manufactured electrical interconnects, where an interconnect comprises a point of contact between at least two different materials. Key to the proper function of such interconnect devices is the robustness of the interconnect and its ability to reliably conduct electronic signals such as current and potential. Additionally, interconnect devices may also be required to conduct photons as for example to transmit light-based signals. Dependable techniques of manufacturing strive to consistently create high-quality, defect-free interconnects. Such devices fail when contact across the interconnect is impeded or prevented. For example, at small dimensions surface roughness at the contact boundary can make it difficult to achieve or maintain contact sufficient to ensure proper electrical conduction. At dimensions approaching the nanometer scale, normal surface topology of metal surfaces ordinarily used in interconnects can prevent large portions of the corresponding surfaces from establishing contact. These gaps substantially increase the electrical resistance in the interconnect device and often result in an interconnect device that cannot adequately conduct electrical current.
Recent advances in nanotechnology have made it possible to consider the smallest possible sizes for electronic devices. Namely, circuits and devices, including electrical interconnects, employing devices that comprise one or a small collection of molecules are now within the realm of plausible device structures. Engineering good contacts at the molecular level poses a significant challenge. As the fabrication of coherent molecular electronic structures on various surfaces evolves, the detailed chemical nature of the connection between the molecular and macro-scale worlds will become increasingly important. See, for example, Cahen, D.; Hodes, G. Adv. Mater. 2002, 14, 789 and Yaliraki, S. N.; Ratner, M. A. Ann. N.Y. Acad. Sci. 2002, 960, 153.
Recent advances in “wet” surface chemistry offer an increasingly sophisticated range of techniques for self-orienting molecular chemisorption on a wide variety of materials. See, e.g., Ullman, A., Chem. Rev. 1996, 96, 1533; see also, Buriak, J. M., Chem. Rev. 2002, 102, 1271; Seker, F.; Meeker, K.; Kuech, T. F.; Ellis, A. B.; Chem. Rev., 2000, 100, 2505. Such techniques expand the broad, general applicability of synthetic chemistry to heterogeneous phase and have improved the prospects for “bottom up” fabrication strategies in nanotechnology. Specifically, work related to the construction of post-CMOS (complementary metal-oxide semiconductor) hybrid electronic devices using chemical techniques and molecular components to augment traditional fabrication techniques requires more control at the molecule/contact interfaces. Ideally, in the case of electronic devices employing conjugated organic molecules, a bond allowing strong electronic coupling between the energy bands of a bulk contact and the orbitals of a conjugated organic molecule would allow for a great deal of synthetic variation in device properties. Recent advances in surface chemistry offer an increasingly sophisticated range of techniques for orienting molecules on a wide variety of materials. See for example, Ullman, A. Chem. Rev. 1996, 96, 1533; Buriak, J. M. Chem. Rev. 2002, 102, 1271; and Seker, F.; Meeker, K.; Kuech, T. F.; Ellis, A. B. Chem. Rev. 2000, 100, 2505. These new techniques improve the prospects of future ‘bottom-up’ fabrication strategies in nanotechnology using chemical techniques and molecular components to augment traditional fabrication schemes. See, for example, Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550; Tour, J. M. Acc. Chem. Res. 2000, 33, 791; and Reed, M. A.; Chen, J.; Rawlett, A. M.; Price, D. W.; Tour, J. M. App. Phys. Lett. 2001, 78, 3735, all incorporated herein by reference.
In many experimental molecular electronic systems, molecules assembled between bulk metallic electrodes have chemical contacts that are highly polar, such as the sulfur-metal bond. This allows for undesired interfacial capacitance, possible electrochemical activity at the bond interface, and generally causes the molecule in the electrode gap to behave as a tunneling barrier. If the electronic properties of various chemical substituents on molecular devices are to be more fully exploited, a less polar, more electronically continuous chemical interface is required.
Some have attempted to functionalize surfaces with organic molecules employing various combinations of conditions and/or reagents.
U.S. Pat. No. 5,429,708 to Linford et al. provides for a method for producing a molecular layer of a selected molecular moiety on a silicon surface in which a silicon surface is etched to form a hydrogenated silicon surface and combined with a free radical-producing compound, where the free radical produced by the free radical-producing compound corresponds to the selected molecular moiety. The combined silicon surface and free radical-producing compound is then heated to sufficient temperature to initiate reaction between the free radical-producing compound and the hydrogenated silicon surface.
U.S. Pat. No. 6,284,317 B1 to Laibinis et al. relates to methods of derivatizing semiconductor surfaces, particularly porous silicon surfaces with silicon-carbon units. The derivatization occurs through the direct addition of an organometallic reagent in the absence of an external energy source such as heat and photochemical or electrochemical energies. The method of the invention allows the formation of unique intermediates including silicon hydride units bonded to metal ions. Because of these unique intermediates, it is possible to form previously inaccessible silicon-carbon units, for example where the carbon atom is an unsaturated carbon atom. Such inaccessible silicon-carbon units also include silicon-polymer covalent bond formation, in particular where the polymer is a conducting polymer. Thus, the present invention also provides a novel semiconductor surface/polymer junction having improved interfacial interactions.
U.S. Pat. No. 6,217,740 B1 to Andrieux et al. concerns a process for electrochemically producing a carbonaceous material with its surface modified by organic groups, in particular functionalized organic groups. The process comprises providing a solution, in a protic or aprotic solvent, comprising a salt of a carboxylate of an organic residue capable of undergoing a Kolbe reaction. The solution is then put in contact with a carbonaceous material, wherein the carbonaceous material is positively polarized with respect to a cathode that is also in contact with the solution. The solution may optionally contain an electrolyte. The invention also concerns carbonaceous materials modified at the surface with arylmethyl groups and the use of these modified materials, for example, in the production of composite materials.
U.S. Pat. No. 5,554,739 to Belmont discloses processes for preparing a carbon product having an organic group attached to a carbon material. The carbon material is selected from graphite powder, a graphite fiber, a carbon fiber, a carbon cloth, a vitreous carbon product, and an activated carbon product. In one process at least one diazonium salt reacts with a carbon material, in the absence of an externally applied electric potential, sufficient to activate the diazonium salt. In another process at least one diazonium salt reacts with a carbon material in a protic reaction medium.
U.S. Pat. No. 6,042,643 to Belmont et al. discloses processes for preparing a carbon black product having an organic group attached to the carbon black. In one process at least one diazonium salt reacts with a carbon black in the absence of an externally applied electric current sufficient to reduce the diazonium salt. In another process at least one diazonium salt reacts with a carbon black in a protic reaction medium. Carbon black products which may be prepared according to process of the invention are described as well as uses of such carbon black products in plastic compositions, rubber compositions, paper compositions, and textile compositions.
PCT Patent Application No. 03/032330 to Tour et al., filed on Jul. 26, 2002 and incorporated herein by reference, describes an electrical interconnect device achieved by applying to the surface of the contact(s) a molecular coating chosen from the group consisting of monomers, oligomers, or polymers that are primarily organic in origin, capable of forming self-assembled monolayers or self-assembled multilayers, electrically conducting or non-conducting, and contain metal-binding ligands as pendant groups or as part of their backbone.
J. Phys. Chem. B 1997, vol. 101, pp. 2415-2420 considers an electrochemical approach to derivatize atomically flat Si(111) surfaces with aryl adlayers. In particular, what is shown is that the electrochemical reduction of 4-nitro- and 4-bromobenzenediazonium salts in an aqueous acidic HF solution under applied external potential leads to modification of Si(111) surfaces.
Polymer 2003, vol. 44, pp 19-24 teaches that reduced polytetrafluoroethylene (PTFE) can be used to graft nitro and bromo-phenyl diazonium tetrafluoroborate salts in a manner similar to that used for carbon, except that no application of a reductive potential during grafting was required.
Notwithstanding the teachings of the prior art, the problem of making a high-quality molecule-surface interface that provides for a bond of sufficient strength and quality to effect good electronic or photonic interaction between an organic molecule and a surface remains less than completely solved. Moreover, a need remains for a method of making a high-quality molecule-surface interface using a minimum of additional steps, reagents or energy. Furthermore, it is believed that a direct covalent bond, allowing stronger electronic coupling between the energy bands of a bulk contact and the frontier orbitals of a conjugated organic molecule, would allow for a greater measure of synthetic variation in device properties and make contact effects less dominant. Furthermore, once the undesirable contact effects have been overcome, it is believed that a hybrid molecular electronic device can be constructed which can find various applications including those of switching, memory and sensing applications.